Rubber composition containing carbon fibrils and a pneumatic tire

ABSTRACT

A rubber composition containing carbon fibrils in which 0.5 to 60 parts by weight of carbon fibril material comprised primarily of an aggregate of fibrils having an average particle diameter of 0.05 to 50 microns in which fine, filiform carbon fibrils of 3.5 to 75 nm in diameter are intertwined with each other is mixed with 99.5 to 40 parts by weight of synthetic rubber and/or natural rubber and a pneumatic tire in which the surface layer is provided with this rubber composition.

FIELD OF THE INVENTION

This invention relates to a novel rubber composition of superiorhardness and strength. More particularly, this invention relates to arubber composition containing carbon fibrils in which specified carbonfibrils are compounded with synthetic rubber and/or natural rubber andto tires in which the composition is used.

BACKGROUND OF THE INVENTION

Concerns over the limited oil resources in recent years has let to theimposition of certain fuel cost standards in an effort to bring aboutimprovement in controlling automobile fuel costs. In order to meet thestrict standards of the future, it will be necessary to improve existingmaterials and technology. One method is to reduce the weight of the tireitself, which would be effective in bringing about great improvement inautomobile fuel costs.

Lowering the specific gravity of the rubber component that forms thetire can be considered. From this standpoint, it would be desirable toeffect a great decrease in the quantity of use of carbon black, whichhas a high specific gravity. Carbon black has approximately twice thespecific gravity as that of rubber. However, when the quantity of carbonis decreased, hardness, tensile strength and modulus during lowextension are decreased and wear resistance is insufficient.

Another way of lowering the weight of the tire that has been consideredis to decrease the size of the tire. However, when the tire is madesmaller, the strength of the tire cannot be maintained. When thequantity of carbon black that is used is increased, hardness isincreased. However, increase in strength is not sufficient. When an everlarger amount of carbon black is used, strength is decreased, theprocessing capacity of the rubber composition becomes poor and heatgenerated from vulcanized rubber is increased. In addition, there arethe problems that the specific gravity of the rubber compositionincreases and the weight of the tire increases.

Consequently, it is desirous to have a reinforcing material which has agreater reinforcing capacity than conventional carbon black and that hasgreater hardness and wear resistance when used in small quantities. Mostrecently, fine, filiform carbon fibrils have been developed and it hasbeen discovered that rubber compositions having hardness and goodtensile strength and wear resistance can be obtained by addition ofsmall amounts of reinforcing material. However, problems such asdeterioration in the physical properties, and, of tensile strength inparticular, of these rubber compositions at high temperatures remain.

OBJECTS OF THE INVENTION

It is therefore a general object of the invention to provide a rubbercomposition that has a low specific gravity.

It is a further object of the invention to provide a rubber compositionhaving a high degree of hardness, tensile strength and modulus.

It is a yet another object to provide a rubber composition having a highwear resistance.

It is another object to provide a pneumatic tire in which the surfacelayer of the tire is composed of a rubber composition having a lowspecific gravity, high degree of hardness, tensile strength and modulus,and a high wear resistance.

These and other objects, features and advantages of the invention willbecome readily apparent from the ensuing description, and the novelfeatures will be particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

The invention is broadly directed to a rubber composition containingcarbon fibrils in which 0.5 to 60 parts by weight of carbon fibrilmaterial comprised primarily of an aggregate having an average particlediameter of 0.05 to 50 μm in which fine, filiform carbon fibrils of 3.5to 75 nm in diameter are intertwined with each other is mixed with 99.5to 40 parts by weight of synthetic rubber and/or natural rubber.

The invention is also broadly directed to a pneumatic tire in which thesurface layer of the tire is provided with the rubber composition asdescribed.

BRIEF DESCRIPTION OP THE DRAWINGS

The invention will be understood more clearly and fully from thefollowing detailed description, when read with reference to theaccompanying figures, in which:

FIG. 1 shows TEM (transmission electron microscope) image of the carbonfibril material used in the manufacture of the rubber composition ofthis invention;

FIG. 2 shows the oxygen of fibril after oxidation;

FIG. 3 shows the ESCA spectra of fibril oxidized by nitric acid invarious time periods;

FIG. 4 shows the ESCA wave separations of C-1s for fibrils oxidized withnitric acid for 20 hours;

FIG. 5 shows the structures of oxygen-containing groups in fibrils;

FIG. 6 shows the change of oxygen content of fibrils;

FIG. 7 shows an XPS spectrum of nitric acid oxidized BN fibrils; and

FIG. 8 shows a high resolution spectrum of the carbon 1s peaks in thespectrum of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

The invention is broadly directed to a rubber composition containingcarbon fibrils and is mixed with synthetic and/or natural rubber.

A rubber composition containing specified carbon fibrils is disclosed inJapanese Patent Disclosure No. 2-235945 1990! as a means of solving theaforementioned problems. That disclosure indicated the diameter of theaggregate of carbon fibrils is 0.10 to 0.25 mm and the rupture strengthT_(B) of the rubber sheet that is obtained is decreased at smallerdiameters.

However, the inventors of this invention, who conducted furtherintensive and repeated research, discovered as a result that thedispersibility of the carbon fibril material and the external appearanceof the compact surface could be improved without impairing the T_(B) andH_(S), even when the average particle diameter of the carbon fibrilaggregate was even smaller. It was further ascertained that theconductive properties of the rubber composition could be improved andthat equal resistance values could be obtained by means of a smallquantity of carbon fibril material.

As a result of research for the purpose of applying the results of theforegoing study industrially, it was discovered that rubber compositionsin which a specified carbon fibril material was compounded in a suitablequantity with any commercial synthetic rubber and/or natural rubber areof superior processing capacity and that they form a vulcanized rubberof high hardness and of superior tensile strength and wear resistance asa result of vulcanization.

The essential aspects of this invention are that it is a rubbercomposition containing carbon fibrils in which 0.5 to 60 parts by weightof carbon fibril material comprised primarily of an aggregate of fibrilshaving an average particle diameter of 0.05 to 50 μm in which fine,filiform carbon fibrils of 3.5 to 75 nm in diameter are intertwined witheach other is mixed with 99.5 to 40 parts by weight of synthetic rubberand/or natural rubber and that pneumatic tire in which the surface layerof the tire is provided with this rubber composition.

Here, the tire surface layer refers to the tread, the side wall orvarious types of covering rubber. In tread that is comprised of severallayers, the tread includes not only the outside layer of tread, but alsothe inside layer of tread that is exposed on the surface of the tireafter it has run. In this invention, the aforementioned rubbercomposition should be provided with a tread.

The term "90% diameter" is used in the explanation of this invention. Itis defined as follows.

The distribution obtained by taking d as particle diameter and thevolumetric ratio of particle diameter Vd as the probability variable iscalled the particle size distribution D. When the minimum particlediameter in the particle size distribution D is taken as dmin and themaximum particle diameter is taken as dmax, the average particlediameter dm satisfies the following formula.

Formula 1: ##EQU1##

In addition, "90% diameter" d₉₀ satisfies the following formula:

Formula 2: ##EQU2##

The carbon fibril material that is used in this invention is comprisedof an aggregate of an average particle diameter of 0.05 to 50 μm inwhich fine, filiform carbon fibrils of 3.5 to 75 nm in diameter areintertwined with each other. The average particle diameter of theaggregate should be 0.2 to 30 μm, and, preferably, 0.5 to 20 μm.

The particle size distribution of the aggregate in this invention is asfollows. Specifically, 90% diameter as defined previously shouldordinarily be less than 100 μm, preferably, less than 80 μm, and, morepreferably, less than 50 μm. Further, 90% diameter should be less than7.5 times the average particle diameter.

When the average particle diameter of the aggregate exceeds 50 m, thecarbon fibril material in the rubber composition is poorly dispersed,the tensile strength of the vulcanized material is decreased and theexternal appearance of the molded compact is impaired. Manufacture isdifficult when the average particle diameter is less that 0.05 m.

The proportion of aggregate in the carbon fibril material should begreater than 30%, and, preferably, greater than 50%.

The carbon fibrils that form the carbon fibril aggregate are filamentsof which the variation in diameter should be less than 15% of theaverage diameter of several tens of samples and of which the aspectratio should ordinarily be greater than 5, preferably, greater than 100,and, more preferably, greater than 1000. Moreover, they shouldordinarily be tubular fibrils with hollow cores.

Moreover, the carbon fibrils should have several graphite layers thatare parallel to the fibril axis and should not have a continuous thermalcarbon coating. The proportion of the surface area that is coated by thethermal carbon coating should ordinarily be less than 50%, preferably,less than 25%, and, more preferably, less than 5%.

The surfaces of carbon fibrils which have been denatured can be used.Denaturing can be performed, for example, by chemical reactions, such asoxidation and by procedures such as coating with polymers, like epoxyresins.

The carbon fibrils that are used in this invention should be partiallyoxidized carbon fibrils in which the relative ration of C_(IS) andO_(IS) (C_(IS) /O_(IS)) as determined by X-ray photoelectronspectrometry is in the range of 92/8 to 98/2. When the ratio is lessthan 92/8, dispersion in the rubber is not sufficient, causing thetensile strength of the vulcanized rubber to decreased. When the ratioof C_(IS) and O_(IS) is greater than 98/2, the tensile strength aftermaintenance at high temperatures is decreased. This type of rubbercomposition is suited for use in automobile tires and passengerautomobile tires in particular.

The proportion of carbon fibril material in the composition of thisinvention should be 0.5 to 60% by weight, preferably, 1 to 50% byweight, and, more preferably, 5 to 40% by weight. In tires, theproportion of carbon fibrils should be 15 to 60% by weight, and,preferably, 20 to 50% by weight. In tires, the proportion of carbonfibrils should be 15 to 60% by weight, and, preferably, 20 to 50% byweight. When it is less than 0.5% by weight, the effect attributable tothe carbon fibril material is not manifested. When it exceeds 60% byweight, there are the drawbacks that the processing capacity of thecomposition is markedly poor and that the hardness of vulcanizedcomposition is excessively great.

The method of manufacturing the carbon fibrils that are used in thisinvention is described in Japanese Patent Application 2-503334 1990!. Aspecific example is described below.

The carbon fibrils are manufactured in a vertical tubular reactor byintroducing catalyst particles containing metal into a gas flowcontaining carbon by means of its own weight or by injection of a gassuch as an inert gas. The reaction temperature is 550° to 1200° C. Thecatalyst particles may be formed in the reaction vessel by decompositionof a precursor compound, for example, ferrocene. An internal plug ofquartz wool for catching the catalyst particles and a quartz tubeequipped with a thermocouple for monitoring the temperature of thereaction vessel are used in the reaction vessel. In addition, it isequipped with an inlet port for introducing the catalyst, the reactiongas and a purge gas such as argon and with an outlet port for removingthe gas from the reaction vessel.

Suitable gases containing carbon include saturated hydrocarbons, forexample, methane, ethane, propane, butane, hexane and cyclohexane,unsaturated hydrocarbons, for example, ethylene, propylene, benzene andtoluene, hydrocarbons containing oxygen, for example, acetone, methanoland tetrahydrofuran and carbon monoxide. The preferable gases areethylene and propane. Preferably, hydrogen gas is added. Typically, theratio of gas containing carbon to hydrogen gas is in the range of 1:20to 20:1. Desirable catalysts include iron, molybdenum-iron,chromium-iron, cerium-iron and manganese-iron particles attached todeposited alumina.

In order to cause the fibrils to grown, the reaction tube is heated to550° to 1200° C., and, at the same time, purging is performed with, forexample, argon. When the reaction tube reaches a specified temperature,introduction of the hydrogen flow and the flow of gas containing carbonis begun. A hydrogen flow volume of approximately 100 millimeters/minuteand a flow of gas containing carbon of approximately 200millimeters/minute are suitable for a reaction tube of 1 inch in length.After the reaction tube has been purged for over 5 minutes with thereaction gases at the aforementioned flow volumes, the catalyst isdropped onto the quartz wool plug. Next, the reaction gases are reactedwith the catalyst (typically for 0.5 to 1 hour) throughout the entirebody of the reaction vessel. When the reaction period is completed, theflow of reaction gases is stopped, purging is effected with a gas notcontaining carbon, for example, argon, the reaction vessel is cooled toroom temperature and the fibrils are recovered from the reaction tube.The yield of fibrils is greater than 30 times the iron content of thecatalyst.

The carbon fibril material that is used in this invention consists ofcarbon fibrils in unaltered form manufactured as described above, or, inmany cases, is obtained by pulverizing them to a specified size. Thepulverization apparatus may be, for example, a pneumatic grinder (jetmill) or an impact grinder. Because these grinders can be operatedcontinuously and the quantity treated per unit time is greater than witha ball mill or a vibrating mill, pulverization costs can be lowered. Inaddition, a uniform carbon fibril aggregate of a narrow particle sizedistribution can be obtained by installing a classifying mechanism inthe grinder or by installing a classifier such as a cyclone in the line.

The partially oxidized carbon fibrils that are used in this inventioncan be manufactured using carbon fibrils as the carbon fibrils and byoxidizing their surfaces. They can be manufactured by gaseous phaseoxidation at normal temperature or high temperatures with such oxidatinggases as air, oxygen, ozone, water vapor and plasma and by liquid phaseoxidation with concentrated nitric acid, potassium permanganate,potassium dichromate and sodium hypochlorite. To prevent environmentalcontamination, gaseous phase oxidation is preferably conducted with anindustrial oxidizing gas. Manufacture can be carried out by combiningthe oxidation treatment process with the process of manufacturing thecarbon fibrils. These oxidation treatments can also be carried out afterthe raw material carbon fibrils have been pulverized.

FIG. 1 shows an example of the carbon fibril material that is used inthis invention. The portions that are shaded in black are the carbonfibril aggregate obtained as described above. The matter that appears aslines is the fibrils themselves.

The synthetic rubbers that are used in this invention includepolyisoprene rubber, polybutadiene rubber, butadiene-styrene copolymerrubber, butadiene-acrylonitrile copolymer rubber, polychloroprenerubber, ethylene-olefinic copolymer rubber, ethylene-acrylic copolymerrubber, ethylene-vinyl acetate copolymer, acrylic rubber,epichlorohydrin rubber, halogenated polyethylenes, cholorsulfonatedpolyethylenes, silicone rubber, fluorine rubber and phosphazene rubber.

Modified substances obtained by adding maleic acid anhydride, α,β-unsaturated carboxylic acids and esters thereof, various types ofvinyl compounds and acenaphthylene to the aforementioned rubbers andmodified substances obtained by hydrogenating those of theaforementioned rubbers having unsaturated groups in the polymer mainchain can also be cited.

Diene polymers, specifically, polybutadiene rubber, butadiene-styrenecopolymer rubber and polyisoprene rubber can be used suitably in tires.

The vinyl content of the butadiene component of the (co)polymer in thebutadiene rubber compound should be greater than 15%, preferably,greater than 20%, and, more preferably, greater than 30%. From thestandpoints of manufacture and effect, it should be less than 90%.

When it is less than 15%, it is difficult to improve wet skidcharacteristics and roll friction resistance characteristics areimproved, roll friction resistance characteristics at the same time.That is, when wet skid characteristics are impaired, and, when rollfriction resistance characteristics are improved, wet skidcharacteristics are impaired.

The glass transition temperature (Tg) of the aforementioned butadiene(co)polymers should be greater than -70° C., and, preferably, greaterthan -60° C. For effectiveness, it should be less than -30° C. When theglass transition temperature is less than said temperature, wet skidcharacteristics are impaired. This is not desirable. The glasstransition temperatures (Tg) indicates values determined by DSC. In thisconnection, the value for Li butadiene rubber of a 12% vinyl bondcontent is -180° C., the value for natural rubber is -76° C. and thevalue for styrene-butadiene copolymer rubber (SBR #1500: brand name)obtained by emulsion polymerization is -64° C.

Conjugated diene (co)polymers that are desirable for use in tires can beobtained by subjecting a conjugated diene alone or a conjugated dienetogether with one or more other conjugated dienes or aromatic vinylcompounds to solution polymerization, after which the product is reactedwith a reactive compound such as an isocyanate compound.

Styrene-butadiene copolymers having a styrene content of greater than 5%are particularly desirable because they exhibit excellent wet skidcharacteristics and roll friction resistance characteristics and becausethey are also of superior tensile strength and processing capacity.

Although there are no particular limitations on the aforementionedstyrene content, it should be less than 50% by weight, and, preferably,less than 45% by weight.

Additives, vulcanization accelerators, auxiliary vulcanizationaccelerators, aging inhibitors, softeners and fillers that are commonlyused in the rubber industry can be compounded with the rubbercomposition of this invention.

Further, as required, fillers such as carbon black, silica, diatomaceousearth, pulverized quartz, talc, clay, mica, calcium silicate, magnesiumsilicate, glass powder, calcium silicate, magnesium silicate, glasspowder, calcium carbonate, barium sulfate, zinc carbonate, titaniumoxide, alumina, glass fibers, other types of carbon fibers and organicfibers and known additives such as softeners, plasticizers, auxiliaryprocessing agents, lubricants, aging inhibitors and ultraviolet rayabsorbents can also be added.

These compounding substances can be kneaded with kneading machines thatare commonly used such as rolling machines and Bumbury mixers, afterwhich molding and vulcanization can be carried out under ordinaryconditions for manufacturing vulcanized rubber.

Mixing of the carbon fibril material of this invention and the rubbercan be carried out by the wet master batch method.

The invention will be more fully described and understood with referenceto the following examples which are given by way of illustration.

In determination of the diameter of the aggregate of the carbon fibrilmaterial, the carbon fibrils were first added to water to which asurfactant had been added and were dispersed using an ultrasonichomogenizer. Following that, the carbon fibril dispersion was analyzedusing a laser diffraction scattering type particle size distributionmeter and the particle diameter of the aggregate was determined.

Compounding for the purpose of vulcanization tests in the examples andcomparative examples was as indicated in Table 1 through Table 3. Theunit of compounding was parts by weight in all cases. Carbon fibrils orHAF carbon (high reinforcing carbon black) were added to the compoundingmaterials based on compounding formulation 1 and compounding formulation2 in Table 1 by compounding as indicated in Table 2 and Table 3. Asspecified in Table 2 and Table 3, the types of rubbers were SBR rubberand EP rubber.

Table 2 and Table 3 also show the properties of the carbon fibrilmaterial and the test results for the examples and comparative examples.

                  TABLE 1    ______________________________________                  Compounding                           Compounding                  1        2    ______________________________________    Rubber (SBR rubber                    100        100    or EP rubber)    Carbon fibrils  *          *    HAF carbon      *          *    (ASTM 330)    Stearic acid    2          1    Zinc white      3          5    N-butyl-N-isopropyl-                    2          --    p-phenylenediamine    Diphenylguanidine                    1          --    Dibenzothiazole 0.6        --    disulfide    N-cyclohexyl-2- --         1.5    benzothiazole    disulfide    Dipentamethylene                    --         0.75    thiuram tetrasulfide    Tellurium diethyl                    --         0.75    dithiocarbamate    Sulfur          1.5        1.5    ______________________________________     Cases marked with * were compounded as indicated in Table 2 and Table 3.

                                      TABLE 2    __________________________________________________________________________                 Example                      Comparative                            Example                                 Comparative                                       Example                                            Comparative                                                  Example                                                       Comparative                                                             Comparative                 1    Example 1                            2    Example 2                                       3    Example 3                                                  4    Example                                                             Example    __________________________________________________________________________                                                             5                 1    1     2    2     1    1     1    1     1    Carbon fibril properties    Diameter (nm)                 13   --    13   --    --   --    13   --    13    Average particle diameter of                 3.5  --    3.5  --    --   --    7.4  --    80    aggregate (μm)    Aggregate 90% diameter (μm)                 8.2  --    8.2  --    --   --    34   --    240    Compounding    Carbon fibrils                 30   --    40   --    3    --    35   --    30    HAF carbon (ASTM N-330)                 --   70    --   40    --   3     --   100   --    SBR rubber   100  100   --   --    100  100   100  100   100    EP rubber    --   --    100  100   --   --    --   --    --    Results of evaluation    Roll processing capacity                 good good  good good  average                                            average                                                  good poor  poor    Properties of vulcanized    material    Hardness (H.sub.S) (JIS-A)                 83   81    93   75    56   45    88   98    79    Tensile strength (T.sub.B)(kgf/cm.sup.2)                 222  220   224  206   102  76    241  110   125    Pico wear (index)                 100  78    --   --    56   --    --   98    --    DIN wear (mg)                 118  135   --   --    --   --    85   --    134    Volumetric intrinsic viscosity                 3.0 × 10                      2.0 × 10.sup.3                            3.1 × 10.sup.3                                 over 10.sup.7                                       1.2 × 10.sup.3                                            over 10.sup.7                                                  1.0 × 10.sup.0                                                       5.8 × 10.sup.2                                                             7.6 × 10    (Ωcm)    __________________________________________________________________________

                  TABLE 3    ______________________________________                                   Compa- Compa-            Exam- Exam-    Exam-   rative rative            ple 5 ple 6    ple 7   Ex. 6  Ex. 1    ______________________________________    Type of   1       1        1     1      1    Compounding    Carbon fibril    properties    C.sub.IS /O.sub.IS              97/3    95/5     97/3  99/1   --    Diameter (nm)              12      12       12    13.5   --    Average   2.9     2.9      2.9   3.2    --    particle    diameter of    aggregate (μm)    Compounding    Carbon fibrils              30      30       40    30     --    HAF carbon              --      --       --    --     75    (ASTM N-30)    SBR rubber              100     100      100   100    100    Results of    evaluation    Roll      good    good     good  good   good    processing    capacity    Properties of    vulcanized    material    Hardness (H.sub.S)              84      85       93    83     85    (JIS-A)    After 48 hours              89      89       96    92     92    at 100° C.    Tensile   226     225      247   223    218    strength (TB)    (kgf/cm.sub.2)    After 48 hours              180     182      181   161    160    at 100° C.    Pico wear 100     100      112   99     81    (index)    DIN wear (mg)              116     112      84    118    133    ______________________________________

EXAMPLE 1

An aggregate of an average particle diameter of 3.5 μm and a 90%diameter as described previously of 8.2 μm in which fine, filiformtubular graphitic carbon fibrils of an average diameter of 13 nm wereintertwined were used as the carbon fibril material.

This was compounded on the basis of compounding formulation 1 andcompounding formulation 2 as shown in Table 1 in accordance with thecompounding conditions described in Table 2. SBR 1502 manufactured byJapan Synthetic Rubber was used as the SBR rubber. The compoundedmaterial was kneaded using a laboplast mill and a roller, after whichvulcanization was carried out for 30 minutes at 145° C., with a rubbersheet of approximately 2 mm in thickness being obtained.

Tests were carried out in accordance with the tensile test method forvulcanized rubber specified in JIS K6301 and the for hardness (H_(S))and rupture strength (T_(B)) shown in Table 2 were obtained. Pico weartests as specified in ASTM D-2228 were carried out wear resistance wasexpressed by an index obtained by taking the values in Table 1 as 100.

COMPARATIVE EXAMPLE 1

Tests were conducted in the same way as in Example 1 except that 70parts of HAF carbon (ASTM No. N-330) was used instead of 30 parts ofcarbon fibrils.

When Example 1 and Comparative Example 1 were compared, it was foundthat hardness and tensile strength equal to that with HAF carbon wasobtained with approximately half the quantity carbon fibrils. Wearresistance was also superior.

EXAMPLE 2 AND COMPARATIVE EXAMPLE 2

Tests were carried out in the same way as in Example 1 except that EPrubber (EP21 manufactured by Japan Synthetic Rubber) was used incompounding formulation 2 in Table 1 and that carbon fibrils and HAFcarbon were used as in Example 1 and Comparative Example 1,respectively.

In EP rubber as well, carbon fibrils were more effective in increasinghardness than HAF carbon. Volumetric intrinsic resistance wasapproximately 1/10,000 in Example 2. Thus, carbon fibrils were extremelygood in increasing conductivity.

EXAMPLE 3

Tests were conducted in the same way as in Example 1, except that 3parts of carbon fibrils was used.

COMPARATIVE EXAMPLE 3

Test were carried out in the same way as in Comparative Example 1 exceptthat 3 parts of HAF carbon was used. From a comparison of Example 3 andComparative Example 3, it was found that carbon fibrils in small amountshad the effect of bringing about marked decreases in the volumetricintrinsic resistance of vulcanized rubber.

EXAMPLE 4

An aggregate of an average particle diameter of 7.4 μm and a 90%particle diameter as described previously of 34 μm in which fine,filiform tubular graphitic carbon fibrils of 13 nm in average diameterwere intertwined was used as the carbon fibril material. This aggregatewas tested in the same way as in Example 1 using compound formulation 1in Table 1 and under the compounding conditions of Table 2.

The quantity of carbon fibrils was increased by 5 parts by comparison toExample 1, for which reason there was high hardness and tensile strengthas well as high conductivity. There was little wear.

COMPARATIVE EXAMPLE 4

Tests were carried out in the same way as in Comparative Example 2except that 100 parts carbon were used. Roller processing capacity waspoor and tensile strength was also low.

COMPARATIVE EXAMPLE 5

An aggregate of an average particle diameter of 80 μm and a 90% particlediameter as described previously of 240 μm in which fine, filiformtubular graphitic carbon fibrils of 13 nm in average diameter wereintertwined was used as the carbon fibril material. This aggregate wastested in the same way as in Example 1 using compound formulation 1 inTable 1 and was compounded as indicated in Table 2. Roller processingcapacity was poorer and tensile strength was lower than in Example 1, inwhich the same quantity of carbon fibrils was compounded. Conductivitywas inferior to that in Example 1.

EXAMPLES 5 TO 7

Concentrated nitric acid was added to the carbon fibrils of Example 1,the materials were heated, a reaction was carried out under reflux andcarbon fibrils of different degrees of oxidation was prepared. Table 3shows their shapes and properties.

These fibrils were compounded as indicated in the column for compoundingformulation 1 of Table 1 and the compounding column in Table 3.Vulcanization and tests of physical properties were carried out inaccordance with the conditions described in Example 1. In addition totests at normal temperature (25° C.), hardness and tensile strength weretested at normal temperature after the test strips were maintained at100° C. for 48 hours.

COMPARATIVE EXAMPLE 6

The tests were carried out in the same way as in Example 5 except that amaterial in which the C_(IS) /O_(IS) ratio was 99/1 was used as thecarbon fibril material and that the other conditions were thoseindicated in Table 3.

COMPARATIVE EXAMPLE 7

The tests were carried out in the same way as in Example 5 except that75 parts of HAF carbon was used instead of carbon fibrils.

When Examples 5 and 6 were compared with Comparative Example 7, it wasfound that there was little change (deterioration) of hardness andtensile strength after maintenance at high temperatures when the carbonfibrils of this invention were used.

When Example 5 and Comparative Example 7 were compared, it was foundthat the carbon fibrils conferred hardness and tensile strength onvulcanized rubber equal to that with HAF carbon when compounded intwo-fifths the quantity of the latter.

EXAMPLE 8

Using CC fibrils, films were cast from the formulations shown in thetable below. The fibrils were first ultrasonically dispersed in theTriton solution, then the latex was added, followed by additionalsonication. The mixture was then dried as a film, after which conductivepaint stripes were applied for resistivity determination.

                  TABLE 4    ______________________________________    All amounts are parts by weight              CONTROLS     TESTS              1    2       1      2     3    4    ______________________________________    Water       2000   2000    2000 2000  2000 2000    Triton X-100.sup.1                --     10      10   10    10   10    BN Fibrils                 --   5     --   10    CC Fibrils                 5    --    10   --    Natural Rubber.sup.2                100    100     100  100   100  100    Resistivity, ohm-cm                ∞                       ∞ 1.1  48    0.3  13    ______________________________________     .sup.1 Triton X100 is a nonionic surface active agent manufactured by Roh     and Haas 2     .sup.2 Added as 161 parts of 62% latex

As can be seen, the rubber containing CC fibrils at 5 parts per hundred(Test 1) is 43 times more conductive than its BN counterpart (Test 2).At the 10 phr loadings (Tests 3 & 4), the conductivity ration isessentially unchanged. Accordingly, it was found that CC fibrilsimparted electrical conductivity substantially more effectively than BNfibrils.

As described above, the rubber composition of this invention exhibitsgood roller processing capacity. Moreover, when the effects of thecarbon fibrils of this invention and carbon black are compared for SBRrubber compositions, essentially the same physical properties in respectto hardness, tensile strength and wear can be obtained with carbonfibrils as with carbon black when the carbon fibrils are compounded inamounts of less than half the carbon black. Volumetric intrinsicresistance values are approximately 1/70th, with vulcanized rubber ofhigh conductivity being obtained.

EXAMPLE 9

Surface modifications of fibrils were conducted. The gas-phaseseparation of geometric isomers of hydrocarbons such as xylenes requestsa tough and strong polymer matrix for membrane formation which does notdissolve in hydrocarbon solvents.

One of favorable candidate polymers for this purpose would be Nylonsince Nylons are stable and tough to hydrocarbon solvents. So, a targetis aimed at Nylon as a matrix in which the fibrils are well dispersed.

In order to have a well-dispersed state of fibrils in Nylon, it would benecessary to modify the surface structure of fibrils through chemicalmodifications. A method for the surface modifications of fibrils wascarried out by surface oxidation with nitric acid by the followingprocedure.

Fibrils were treated under following conditions:

Fibrils: 5 g

36% HNO₃ : 70 ml in 200 mol flask

Heating: reflux at 110° for 5 hrs.

After the treatment fibrils were collected by filtration and washed withwater, repeatedly and dried.

Surface structures of fibrils were characterized by ESCA analyses. Thecontent of oxygen on the fibrils surface increased from 4.35% to 12.42%.It is expected that the increase of the oxygen content might be due tothe formation --COOH group on the surface of fibrils.

EXAMPLE 10

Further studies on surface modification of fibrils by oxidation withnitric acid were carried out under various conditions so that carboxylicacid groups were incorporated onto the surface. At the initial stage ofthe oxidation ether-type oxygen was incorporated and then followingoxidation reactions took place to produce carbonyl and carboxylic acidmoieties.

Surface analyses by ESCA indicate that the incorporation of oxygenreached an equilibrium after 5 hr. treatment in 60% nitric acid at 110°C.

Surface modification of fibrils was carried out by means of oxidationwith concentrated nitric acid so that carboxylic acid groups wereintroduced on the surface of fibrils. Oxidation reaction was carried outas follows: 10 g of fibrils were suspended in 200 ml of 60% nitric acidwhich were heated under reflux conditions at 110° C. During the reactionnitrogen dioxide gas was evolved which was neutralized with 5% aqueouspotassium hydroxide solution. After a given time period of the reaction,the suspension was poured into 21 of water and the fibrils were filteredoff. The oxidized fibrils were washed repeatedly with water, followed bywashing with acetone and drying under vacuum at 50° C. Drying fibrilsdirectly after washing with water yielded a crammed mass which wasdifficult to disperse. Therefore, the washing with acetone, followed byhexane was necessary to keep fibril structure.

Surface analyses of fibrils was carried out by means of ESCA(XPS), usingJEOL Surface Science SSX-100, and also by means of elemental analyses.

FIG. 2 indicates the total oxygen content of fibrils after theoxidation. The initial content of oxygen of fibril was 1.4%, whichincreased gradually by the oxidation. The oxygen content of fibrilsincreased up to 6.8% after one hr oxidation and reached an equilibriumcontent of about 11% after ten hr.

Structural analyses of oxygen were carried out by C-1s wave analyseswhich are shown in FIGS. 3 and 4, as ESCA spectra. Results of thestructures of oxygen are summarized in FIG. 5, which revealed thatether-type oxygen --O--C was observed at the initial state, followed bygradual increases in C═O and --COOH groups.

Presumably, peroxide groups would be formed at the initial stage of theoxidation on the surface of the fibrils, which then transformed intocarbonyl and carboxylic acid groups.

The oxygen contents and the structures of oxygen groups did notsignificantly change after keeping fibril for almost three months inair, as shown in FIG. 6.

EXAMPLE 11

XPS spectrum was conducted on nitric acid oxidized fibrils. FIG. 7 is aXPS spectrum of nitric acid oxidized BN fibrils. It shows carbon 1s andoxygen 1s peaks. The calculated oxygen content from this spectrum is20.6% (atomic percentage). The fibrils were oxidized in 35% nitric acidat 107° C. for 48 hours. FIG. 8 was recorded from the same sample. It isa high resolution spectrum of the carbon 1s peaks. The peaks at 283.94,285.66 and 287.99 eV can be assigned to C, C--OH and COOH (C═O),respectively.

Rubber of high hardness can be obtained by means of the rubbercomposition of this invention without impairing processing capacity andrubber elasticity. Consequently, stability of properties can bemaintained over long periods. The reason for this is that the rubbercomposition of this invention makes it possible to maintain hardnesswhile decreasing the quantity of fillers and sulfur compounded for thepurpose of increasing hardness.

Rubber compositions having sufficient hardness so that they can be usedfor radial tire treads of large vehicles while increase in heatgeneration by the rubber is avoided can be obtained more readily thanwith high reinforcing carbon black which is compounded forreinforcement.

When partially oxidized carbon fibrils are used, hardness, tensilestrength and wear resistance are of the same degree as with carbon blackwhen the quantity of carbon compounded is approximately two-fifths thatwith carbon black. Moreover, there is little thermal deterioration ofthe vulcanized rubber.

Thus, the rubber compositions of this invention satisfy performancerequirements which have been becoming higher and higher in recent years,their performance as products is stable over long periods and they canbe used effectively over an extremely broad range of industrial fields.For example, they can be used for automobile components, tirecomponents, rubber rollers, rolling, pads and oil seals.

Having thus described in detail preferred embodiments of the presentinvention, it is to be understood that the invention defined by theappended claims is not limited to particular details set forth in thisdescription as many variations thereof are possible without departingfrom the spirit or scope of the present invention.

What is claimed is:
 1. A rubber composition containing carbon fibrils inwhich 0.5 to 60 parts by weight of carbon fibril material comprisedprimarily of an aggregate of fibrils having an average diameter of 0.05to 50 μm in which fine, filiform carbon fibrils of 3.5 to 75 nm indiameter are intertwined with each other is mixed with 99.5 to 40 partsby weight of synthetic rubber and/or natural rubber.
 2. A pneumatic tirein which the surface layer of the tire is provided with a rubbercomposition, wherein said rubber composition contains carbon fibrils inwhich 0.5 to 60 parts by weight of carbon material comprised primarilyof an aggregate of fibrils having an average diameter of 0.05 to 50 μmin which fine, filiform carbon fibrils of 3.5 to 75 nm in diameter areintertwined with each other is mixed with 99.5 to 40 parts by weight ofsynthetic rubber and/or natural rubber.